Wavelength multiplexers and demultiplexers are used in optical-fiber transmission technology to increase the transmission capacity. For this purpose, the modulated radiation from a plurality of light sources of different wavelengths are transmitted through an optic fiber concurrently. To minimize losses when combining and separating the various wavelength channels at the beginning and end of the transmission path, multiplexers and demultiplexers are known that utilize either interference filters or diffraction gratings. A particularly compact and small configuration is achievable through the use of slab waveguides.
From the journal "Applied Optics", Volume 19, Number 21, (1980), pages 3588 to 3590, a multimode slab waveguide demultiplexer is known using a concave grating. The input fiber has a core diameter of 60 .mu.m, and the output fibers have a core diameter of 250 .mu.m. To avoid coupling losses, the guide layer of the slab waveguide is chosen to be 80 .mu.m thick, thus being greater than the core diameter of the input fiber and smaller than the core diameter of the output fibers.
In a later edition of "Applied Optics", Volume 21, Number 12, (1982), pages 2195 to 2198, a multimode slab waveguide demultiplexer is described which uses a chirped grating. The core diameter and numerical aperture of the optic fibers approximately coincide with those of the slab waveguide.
With these known multimode slab waveguide demultiplexers, sufficient channel separation is only achievable at a channel spacing from 30 nm to 50 nm which corresponds to a wavelength range from 0.8 .mu.m to 1.3 .mu.m, approximately. With a decrease in the channel spacing, a channel overlap occurs because of the propagation of the light in the multimode slab waveguide; however, this channel overlap reaches such a magnitude that the channel separation is no longer sufficient for applications in communications technology.